Electrophotograph developing apparatus

ABSTRACT

In liquid electrophotographic developing apparatus wherein the electrophotographic material is passed between a series of roller pairs, in at least the first roller pair the roller facing the latent image on the electrophotographic material is of an insulating material or coated with an insulating material, the remaining roller pair(s) being development rollers.

1 June 11, 1974 United States Patent [19 Sato et al.

.. ll8/D1G. 23

. 118/637 95/94 117/37 LE l18/DlG. 23

1/1967 Roeber 6/1967 Oliphant et al. 10/1967 Ostensen...... 2/1968 Matkan et al 683 l0/l968 Jons et al..... 101 Smith...........

Miller 8/1972 Sato et al.....

Prim

[22] Filed:

my ExaminerHenry S. Jaudon Assistant Examiner-Leo Millstein AppL No; 201,644

Attorney, Agent, or Firm-LT. Martin; Gerald J. Fer- Joseph J. Baker guson, Jr

Foreign Application Priority Data Nov. 25, 1970 ABSTRACT Cl........ 1n liquid electrophotographic developing apparatus wherein the electrophotographic material is passed between a series of roller pairs, in at least the first roller pair the roller facing the latent image on the electro- 30%,. m he m3D 8 1 E, 6 9 L 5 7 .Q B 8 7 7 m 3 6 n" .u m 1 mm r um um d Ld .mF

117/37 LE; 355/10; 95/89 R, 89 L' photographic material is of an insulating material or References Cited UNITED STATES PATENTS coated with an insulating material, the remaining roller pair(s) being development rollers.

3,155,546 Dirks ll8/D1G. 23 7 Claims, 3 Drawing Figures ELECTROPIIOTOGRAPH DEVELOPING APPARATUS BACKGROUND OF THE INVENTION This invention relates to electrophotograph developing apparatus using a liquid developer, and more particularly it relates to the improvements in such apparatus in which the electrophotographic material sheet is continuously fed while seized by plural pairs of rollers.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view of an embodiment of a conventional developing device in which'the electrophotographic material is fed while being seized or held by roller pairs. I

FIG. 2 shows the distribution of electric charges on the electrophotographic material and the distribution of the electric field in a direction vertical to the latent image surface.

FIG. 3 is a schematic sectional view of an embodiment of apparatus according to the present invention.

DESCRIPTION OF THE PRIOR ART FIG. 1, as indicated, shows a schematicsectional view of a conventional developing apparatus in which the photographic material is fed while being seized by roller pairs. Such apparatus is, for instance, disclosed in Japanese Patent Patent Publication No. 41-9476. In the figure, numeral designates an electrophotographic sheet material having'an electrostatic latent image thereon, and 11 and 12 denote, respectively, developing rollers disposed on opposing faces of the electrophotographic material sheet. Cooperating pairs of rollers are made of metal and are adapted to feed the photographic material sheet by seizing or holdinglthe latter in the manner as shown.

The rollers 11 are adapted to double as developing electrodes, and may be in contact with or slightly spaced apart from the latent image surface. Numeral l3 denotes the nozzles for supplying the liquid developer to the latent image surface. Theliquid developer is first sprayed against the rollers 11. The roller pairs are driven in the directions indicated by arrows by a suitable driving means not shown. Although the apparatus shown in FIG. 1 is excellent in many respects for use as an electrophotographic developing apparatus using a liquid developer, it has the following defects. When an electrophotographic material sheet 10 having an electrostatic latent image on its upper side is inserted in the direction of arrow 15 between the first roller pair 11-1 and 12-1, the latent image face confronts the developing electrode (roller l11) immediately after the start of the developing operation, and when the developer flows into the small space between the developing electrode and the latent image surface immediately after start of the development process streaks are often produced in the image downstream of the flow. Such streaks are more liable to be produced as the flow of the developer increases, as the space between the developing electrodes and the latent image is reduced or as the potential of the latent image is increased.

It has been found, however, that such streaks are less likely to be produced when the developer is first di- 2 rectly applied to the latent image surface outside the immediate area of the developing electrodes, that is, without allowing the image surface to be in close proximity to the developing electrodes immediately after start of development, and the latent image is permitted to develop to the halfway point. Thereafter, the supply of developer to the latent image surface can be convector ducted while the image surface is in close face-to-face relationship with the developing electrodes. This process, however, leads to the results as will hereafter be described with reference to FIG. 2.

FIG. 2 shows the electric charge distribution (A) in the latent image, the electric field distribution (B) in a direction vertical to the latent image surface when no developing electrode is present, and the electric field distribution (C) in a direction vertical to the latent image when the developing electrodes are present. The axis of the abscissa represents the position along a straight line on the electrophotographic material sheet. It is assumed that the electric charges are present uniformly over the entire range on the left side of the origin 0, that the developer is flown from left to right as shown by arrow 20, and that the latent image has the positive charges and the toner has the negative charges. When the developing electrodes are brought in close proximity to the latent image surface simultaneously with the'start of development, the electric field exhibits the characteristic shown by curve C. This shows that there exists a large force effecting the toner in the area where the electric charges are present, while there exists only a small repellant force acting on the toner in the area where the electric charges are absent. Since both the electrostatic force on the latent image and the force of the liquid flow act on the toner, the latter has a velocity vector Ve in the direction where the toner is attracted vertically to the image face, and a velocity Vs in the direction of the liquid flow. Thus, the toner has the velocity of the resultant vector, The direction of this resultant vector is not perpendicular to the latent image surface. Therefore, the toner is deposited not only in the area where the electric charges are present, but also-in the areas where such charges are not present.

Although a toner-repelling electric field exists in the area where electric charges are absent, such repellent electric field is extremely reduced if the developing electrodes are present. Thus, thou h the toner rushes into this area with velocity vector v, the direction of the vector V is changed by the repelling electric field. However, if such a repelling electric field is not so strong as to cause substantial change of the direct i on of the vector V, and therefore, if its velocity vector Vshas a component vertical to the latent image surface and directed toward said surface, the toner will be deposited even in the area where no electric charge is present. This results in producing streaks in the image. It is believed that the above described phenomena are responsible for the appearance of streaks in the image at start of development when developing electrodes are used.

When development is conducted without initially using any developing electrode, the electric field distribution in the image surface presents a tendency as shown by curve B. As will be noted, no toner is depos ited in the area where no electric charge is present, due to the presence of the strong repelling electric field in the area where the electric charges are present. However, if no developing electrode is used throughout the developing operation, it is practically impossible to effect development true to the electric charge distribution because the electric field distribution has a pattern different from that of the electric charge distribution. It is therefore advisable to use the developing elec trodes from a point about midway in the development process. The optimum time at which the developing electrodes are used cannot be definitely stated as it varies according to the particular developer and the photographic material used, but experimentally it has been found preferable to use the developing electrodes after reaching the stage where the toner is sufficiently deposited in the area where the electric charges are present and there remains almost no force which acts to attract the toner to the latent image surface. Such operation is conducted on a limited scale in the apparatus of FIG. ll, i.e., the developer is supplied on the latent image surface with no developing electrode being present. However, such a situation lasts only for short period since the electrophotographic material is moving in the direction of arrow 15 and is therefore immediately brought into a face-to-face relationship with the first developing electrode (roller ll-ll).

SUMMARY OF THE INVENTION According to the present invention, to obtain excellent development results the time in which the development is conducted in the absence of the first developing electrode is preferably within the range of from about 2 to about seconds. A shorter time than the above range may give rise to streaks, while a longer time, though producing no streaks, will elongate the total process time. If total process time is of no concern, the developing operation may be conducted without the use of any developing electrode for an even longer period.

In FIG. 3, there is shown a sectional view of an em bodiment of the apparatus used in practicing the method of the present invention. Rollers 3l-.-1 and 312 are the rollers which characterize the present invention. Rollers 3ll-l and 31-2 are either made of an insulating material or formed by coating the surface of a metal roller with a highly insulating material so that they will not serve as developing electrodes. Rollers 32-1 and 32-2 correspond to roller ll2-l, and are metal rollers.

Rollers 31-1 and 311-2 may'be made of insulating plastics such as acrylic resins, polyamides, polyvinyl chlorides, polyethylene fluorides or the like, or may be formed by coating a metal roller with such an insulating resin.

In theory, both rollers 3l-ll and 31-2 may be metal rollers with no coating thereon, provided that they are arranged with a sufficiently large spacing between them and the latent image surface. However, such arrangement invites leakage of the developer from the space between the roller 3ll-l and the latent image surface, and the liquid that leaks out may flow onto the electrophotographic sheet material in a direction contrary to the traveling direction of said sheet material or may flow over the outside of the apparatus. It will thus be noted that the roller 3lll is so designed as to perform the role of preventing the developer from flowing away from contact with the developing sheet. Roller 3l2 may be made of metal and arranged with an accordingly increased spacing from the latent image surface.

In such case, the spacing should preferably be more than 2 mm.

In FIG. 3, letter L, indicates the area where develop ment is conducted without using developing electrodes and letter L; the area where development is conducted using developing electrodes. It is possible to employ a greater number of insulating rollers. The greater the number of insulating rollers the faster the velocity at which the electrophotographic sheet material is fed.

The present invention is thus directed towards the use of a roller made of insulation or an insulation coated roller so that the development is carried under no development or under a negligible electrode effect at the initial development stage. The roller 3l-1 can thus be made of an insulating material and roller 32-1 can be made of metal or an insulating material.

DETAILED DESCRIPTION OF THE INVENTION The following is an example of the practice of the present invention with an initial comparison to the prior art. I

When an apparatus such as is shown in FIG. 1 was used with no provision of insulating roller, streaks were produced in the image when the electrophotographic sheet material was fed at a velocity of more than 1.5 cm per second. With the apparatus of FIG. 3, no streaks were produced even when the sheet material was fed at a velocity of 7.0 cm per second.

The electrophotographic sheet material used was prepared by coating a paper sheet with a kneaded mixture of photoconductive zinc oxide powder and an insulating resin. Rollers l1, l2 and 32 were stainless steel rollers having a diameter of 18 mm, and rollers 31 were formed of acrylic resin having a diameter of 18 mm.

No streaks were also produced under the same conditions for the case wherein rollers 31 were prepared by coating a 16 mm diameter stainless steel roller with 1 mm thick layer of Teflon (trademark for a polyfluorocarbon product produced by Du Po'nt Chemical Co.).

The developer was prepared by suspending fine charged particles in an insulating solution formed of non-polar hydrocarbons and halogenated hydrocarbons. To be more definite, the developer was prepared by suspending carbon black in kerosene and dissolving therein as a dispersion stabilizer a resin material which is soluble in kerosene. The electrophotographic material was negatively charged while the toner was positively charged. The space between each adjoining roller was 4 mm. When 4 pairs of insulating rollers were used, no streaks were produced even when the feed velocity of the electrophotographic material was more than 10 cm/sec.

In the embodiment of FIG. 3, six pairs of rollers are used, but they may be increased in number as the occasion demands. Usually, it is advisable to make half of the roller pairs insulating and to use metal rollers for the remaining half.

A preferred embodiment of the present invention comprises a roller assembly consisting of 8 to 10 pairs ofinsulating developing rollers and 8 to 10 pairs of metallic developing rollers, the rollers being arranged so that the space between each roller and the latent image surface is about 0.1 to about 0.5 mm. According to this embodiment, it is possible to carry out developing at an electrophotographic sheet material velocity of 5 to 10 cm/sec.

- in outer diameter.

When the roller which is facing the latent image is a metal roller, the roller works as development electrode if the roller is insulated or not from the other components. The roller on the opposite side of the sheet may be a metal roller or a roller made of insulation, though it is preferably a metal roller due to the developing speed.

Also, although in the apparatus shown in FIG. 3 the process is continuous from area (L,) where the development is conducted without electrodes to area (L where development is conducted using developing electrodes, such a transfer may be effected in a stepwise manner. For instance, the thickness of the insulator on the rollers may be gradually reduced or the volume resistivity of the insulatormay be gradually reduced in the direction of the sheet flow. The end effect is thus to incrementally increase the developing electrode effect. It is also possible to use an insulating roller for only the first roller, while using metalrollers for all of the remaining rollers but arranging the metal rollers such that the space between the latent image surface and said rollers gradually decreases. It will further be apparent to one skilled in the art that the passage of the electrophotographic sheet material may be either rectilinear or curvilinear during development.

Other advantages of the present invention include those mentioned below. In the case where development is conducted under conditions where no developing electrode is present up to the halfway point, no insulating roller will be needed in that area. For example, roller 31-2 may be omitted in FIG. 3. In such a case, however, there is the possibility that the sheet will be floated up between the rollers 31-1 and 11-1, causing the latent image face to hit against the surface of the roller 11-]. Such a problem tends to take place particularly when the electrophotographic material sheet curls its latent image surface toward the outside. According to the present invention, which is provided with insulating roller 31-2, the sheet is seized by said roller before the end of the sheet floats up to any troublesome degree, and this problem is not encountered.

The roller 11-] works as a development electrode, that is, the roller pair 11-1/ 12-1 is the metal roller pair. The roller pair 31-2/32-2 is not necessary when the distance between the roller pairs 31-1/32-1 and 11-1/- 12-1 is small, so far as the sequential proceeding of the electrophotographic materials are not obstructed between these roll pairs. The roller pairs also work as guides for the materials.

-Numerous modifications of the invention will become apparent to one of ordinary skill in the art upon reading the foregoing disclosure. During such a reading it will be evident that this invention provides unique,

electrophotograph developing apparatus for accomplishing the objects and advantages herein stated.

What is claimed is: 1. In developing apparatus in which a liquid developer is supplied to the latent image surface on an electrophotographic sheet material while advancing the electrophotographic sheet material by plural roller pairs disposed on opposite sides of said electrophotographic sheet material, the developer being supplied to all of the rollers on the side of the electrophotographic sheet material bearing the latent image, the rollers on the non-latent image side of the sheet material being either conductive or insulative, the improvement comprising at least one roller pair wherein an insulating roller or an insulation-coated conductive roller is one roller of at least the first of the said plural roller pairs encountered by said electrophotographic sheet material, said one roller facing the side of the electrophotographic sheet material bearing the latent image for emphasizing the edge effect in the developed image to thereby minimize streak formation as the electrophotographic sheet material is advanced by the plural roller pairs, the remaining roller pairs being developing electrodes, each including at least one roller having an electroconductive surface facing the side of the electrophotographic sheet material bearing the latent image for minimizing said edge effect whereby development with negligible development electrode effect occurs both before and after said one roller of the insulating roller pair and whereafter development in the presence of said developing electrodes occurs.

2. The apparatus of claim 1 wherein a'number of said plural roller pairs succeeding said one insulating roller pair each include one insulating roller or insulationcoated conductive roller facing the latent image side of said electrophotographic sheet material so that development with negligible development electrode effect occurs at all of said last mentioned number of said plu- .ment electrode effect occurs at all of said first half of the plural roller pairs.

4. The apparatus of claim 3 where there are a total of 16 to 20 of said roller pairs.

5. The apparatus of claim 4 wherein the rollers facing the latent image are spaced between about 0.1 and about 0.5 mm therefrom.

6. The apparatus of claim 1 wherein the conductivity of the plural roller pairs is incrementally increased at least over a portion of the distance between the first and last of the plural roller pairs in the direction of movement of the-electrophotographic sheet material.

7. The apparatus of claim 1 wherein there is only one said insulating roller pair, the remaining roller pairs being said developing electrodes andwhere the spacing between the image bearing surface and the rollers of said developing electrodes facing the latent image bearing surface gradually decreases in the direction of movement of the electrophotographic sheet material. 

1. In developing apparatus in which a liquid developer is supplied to the latent image surface on an electrophotographic sheet material while advancing the electrophotographic sheet material by plural roller pairs disposed on opposite sides of said electrophotographic sheet material, the developer being supplied to all of the rollers on the side of the electrophotographic sheet material bearing the latent image, the rollers on the non-latent image side of the sheet material being either conductive or insulative, the improvement comprising at least one roller pair wherein an insulating roller or an insulation-coated conductive roller is one roller of at least the first of the said plural roller pairs encountered by said electrophotographic sheet material, said one roller facing the side of the electrophotographic sheet material bearing the latent image for emphasizing the edge effect in the developed image to thereby minimize streak formation as the electrophotographic sheet material is advanced by the plural roller pairs, the remaining roller pairs being developing electrodes, each including at least one roller having an electroconductive surface facing the side of the electrophotographic sheet material bearing the latent image for minimizing said edge effect whereby development with negligible development electrode effect occurs both before and after said one roller of the insulating roller pair and whereafter development in the presence of said developing electrodes occurs.
 2. The apparatus of claim 1 wherein a number of said plural roller pairs succeeding said one insulating roller pair each include one insulating roller or insulation-coated conductive roller facing the latent image side of said electrophotographic sheet material so that development with negligible development electrode effect occurs at all of said last mentioned number of said plural roller pairs.
 3. The apparatus of claim 1 wherein about the first half of the said plural roller pairs encountered by said electrophotographic sheet material include an insulating roller or insulation-coated conductive roller facing the latent image side of saiD electrophotographic sheet material so that development with negligible development electrode effect occurs at all of said first half of the plural roller pairs.
 4. The apparatus of claim 3 where there are a total of 16 to 20 of said roller pairs.
 5. The apparatus of claim 4 wherein the rollers facing the latent image are spaced between about 0.1 and about 0.5 mm therefrom.
 6. The apparatus of claim 1 wherein the conductivity of the plural roller pairs is incrementally increased at least over a portion of the distance between the first and last of the plural roller pairs in the direction of movement of the electrophotographic sheet material.
 7. The apparatus of claim 1 wherein there is only one said insulating roller pair, the remaining roller pairs being said developing electrodes and where the spacing between the image bearing surface and the rollers of said developing electrodes facing the latent image bearing surface gradually decreases in the direction of movement of the electrophotographic sheet material. 